It is known that 2,2-dimethyl-3-hydroxypropanal can be prepared from isobutyraldehyde and formaldehyde by aldol addition and that this hydroxyaldehyde can then be hydrogenated to the corresponding diol. Basic compounds are used as catalysts; alkali metal hydroxides, alkaline earth metal hydroxides, and alkali metal carbonates being commonplace. Amines, in particular tertiary mono- or polyamines such as diamines, have also been used. Such a process is described, for example, in German Patent 19 57 591 B2. To prepare 2,2-dimethylpropane-1,3-diol, isobutyraldehyde and formaldehyde are reacted in the presence of tertiary amines and the reaction mixture obtained is hydrogenated. The following, inter alia, are mentioned as suitable: trimethyl-, triethyl-, methyldiethyl- and methyldiisopropyl- and tributyl amines.
The use of amines as addition catalysts has the advantage that only a minor amount, if any, of by-products which are formed when other basic catalysts are used are formed from isobutyraldehyde and formaldehyde. In contrast, attention must be paid to the complete removal of the amines, because diol contaminated with even only a small amount of amines is unsuitable for many intended uses. The preparation of pure 2,2-dimethylpropane-1,3-diol thus requires a very high separation expenditure.
According to German Patent 36 44 675 Al, it is possible to remove the tertiary amine from the diol completely if tri-n-propylamine is employed as the addition catalyst and the crude product is distilled in the presence of isobutanol. Both primary and secondary amines accompany the tertiary amine from the preparation process or are formed from the tertiary amine used as the catalyst during hydrogenation of the addition product. Moreover, basic products are formed in the course of the reaction by reaction of the primary and secondary amines with isobutyraldehyde and/or formaldehyde. The foregoing products and amines are not removed, or only removed partially, from the reaction mixture.